1. Field of Invention
The present invention relates generally to an advanced neck design for cable ties.
2. Description of Related Art
Cable ties have traditionally been formed by an integral molding of a cable tie head and a cable strap. Most of these cable ties involve a design in which the elongated strap extends straight outward from the cable tie head. An example of such a conventional cable tie design is U.S. Pat. No. 3,949,449 to Caveney et al., which is represented in FIGS. 1-3.
As shown, this conventional cable tie 100 includes an elongated strap 110 extending from an integrally molded cable tie head 120. A neck area 130 is formed at the interface between strap 110 and head 120. Cable tie head 120 includes a strap accepting channel 140 that receives strap 110 and a locking device 150. In use, cable tie 100 can be wrapped around objects such as a bundle of cables 160 and locked in place as known in the art.
With such a conventional design, there is often little or no bending at the neck area 130. Rather, bending incrementally occurs throughout the length of strap 110 as apparent from FIG. 3. Most designs for this type of cable tie, including the Caveney ""449 patent, have a uniform strap width B that is substantially smaller than a cable head width E. There may be a slight radiusing at the transition with cable head 120, but for the most part the neck area 130 in such designs has had the same width B and cross-section as the remainder of strap 110.
There also is known a bent neck type of cable tie, such as the one shown in FIGS. 4-9 described in copending U.S. patent application Ser. No. 09/855,262. now U.S. Pat. No. 6,530,126 the disclosure of which is incorporated herein by reference in its entirety. In such a bent neck design, cable tie 100 is again integrally formed with both a cable tie head 120 and a strap 110. However, in this design, strap 110 initially extends from head 120 along a strap attachment axis S substantially parallel to the strap passageway, and is then formed with a bend at neck section 130 such that the strap extends substantially perpendicular to the strap attachment axis S. With such a bent neck design, a more favorable position of the portion of strap 110 exiting the strap passageway after threading is achieved. This can be particularly important when the excess strap length is cut off so as to avoid a sharp edge sticking up. However, a substantial amount of the bending forces acting on cable tie 100 during use act at the bent portion. That is, to accommodate either a very small bundle of cables or a large bundle of cables, strap 110 will need to be stretched inward or outward and the forces from such stretching are concentrated at the prebent neck section 130.
The design shown in FIGS. 4-9 substantially conforms to the conventionally used notion that the neck should correspond in size to the strap. That is, strap 110 has a substantially constant overall width and thickness. Neck section 130 in this design has substantially the same width B as the strap 110 and may include a cored out bottom portion 115 that has a reduced thickness at central portions as best shown in FIG. 7. This reduction in thickness is used to increase flexibility in the neck area. However, there are several problems that may exist with such designs. First, there may be a difficulty in bending at the neck section if the neck section is not cored out. Second, there is a reduced strap strength (tensile strength) when the neck section is cored out compared to the tensile strength of the rest of the strap body. Third, there may be molding and reliability problems. Molding in such a design is achieved by a two-piece mold having a complex shape. The mold has a stepped part line (P/L) as shown with the bolded dashed line in FIGS. 6 and 8 in which the part line follows the midline of strap 110 around the bend of neck section 130 where the part line then angles down across cable tie head 120. However, for the simplest mold tooling design of a cable tie head 120 that has a width E substantially larger than the width B at the neck area (FIG. 4), there is a sharp edge and/or mismatch on the plastic part at the interface between neck section 130 and cable tie head 120. This sharp edge and mismatch can be avoided with complicated tooling, including the complicated metal mold 200 partially shown in FIG. 8. Such complicated tooling, however, has sharp edges that could be easily worn or broken.
Furthermore, for either design there is a sharp step (change in cross-section) from the transition of neck section 130 to cable tie head 120, there is a distinct potential for stress risers. As the neck section bends considerably in either direction, such stress risers can lead to part failures, particularly when brittle materials are used for the part.
It is an object of the invention to provide a cable tie with an improved neck design. The neck design allows improved flexibility and bending at the neck without reducing loop strength from that attained in the strap section. Moreover, the neck design maintains a generous edge radius that prevents cable insulation damage and allows use of a simpler mold design.
Applicants have found that the lie of a strap in a bent neck type cable tie can be improved by maximizing the radius of the neck at the bend and by improving the neck""s flexibility. The strap thickness, parting line angle, and strap body edge radii limit the size of the radii that can transition from cable tie head to neck.
The general equation for the moment of inertia for a simple rectangular cross-section cable tie is I=(Bxc3x97T3)/12 where B is the width of the strap and T is the thickness of the strap. The flexibility of the neck can be improved by: 1) making the strap narrower; or 2) making the strap thinner. Reducing either variable and holding the other constant would reduce the moment of inertia and thereby decrease the force to bend the part in that region. However, doing so will also decrease the area through the section (A≈Bxc3x97T), which has the adverse effect of decreasing tensile strength through the section. As such, attempts to increase bending flexibility using these methods would result in reduced tensile strength, which is undesirable.
Applicants have noted that by increasing the width of the strap while decreasing the thickness, one can achieve a desired lower moment of inertia while maintaining or increasing the tensile strength (area) of the section. The magnitude of the moment of inertia can be decreased as the strap width increases by reducing the overall thickness of the part, or by creating a channel on either or both sides of the part. The channel-shaped geometry has the added advantage that a larger thickness flow path is maintained for the purpose of more easily filling out a molded part.
Applicants have also found manufacturing and use advantages to increasing of the width of the strap to match the width of the cable tie head. This provides the most desirable tooling configuration for a stepped parting line part, which may be used to create bent neck type cable ties such as those of the claimed invention. That is, a neck width that matches the head width eliminates the need for weak or complicated mold components to eliminate sharp edges or mismatches on the part. Such a design in which a neck width matches the head width also eliminates a traditional stress concentration where there is an abrupt change in cross-section.
The above and other objects are achieved by a cable tie that includes an integral cable tie head and strap. The strap includes a first end forming a neck section, a free end opposite the first end, and an intermediate section between the first end and the free end, the intermediate section having a predetermined width B and thickness T1 defining a predetermined cross-sectional area. The cable tie head is secured to the neck area of the strap at the first end of the strap, the cable tie head having a width E that is wider than strap width B and including a strap accepting channel containing a locking device. The strap accepting channel is sized to receive the free end of the strap. The neck section has a width that transitions from a width of B to a width Exe2x80x2 that is substantially the same as width E and a thickness T2 that is thinner than T1, the neck section having a cross-sectional area that is at least substantially equal to the cross-sectional area of the intermediate section of the strap so as to have a tensile strength at least equal to a tensile strength of the intermediate section of the strap. The cable tie may be a bent neck type cable tie. Preferably, the neck section has at least one recessed channel defining the reduced thickness T2 and thickened side portions.